Our hypothesis is that some of the injurious effects of ethanol on the liver are due to the covalent binding of acetaldehyde to hepatocellular macromolecules, thereby interfering with cellular function. The initial objective of this study is to determine the extent of covalent binding of metabolically derived acetaldehyde to liver proteins. Acetaldehyde is a highly reactive compound which can react with a variety of biological substances especially those containing free sulfhydryl or amino groups. Since generation and subsequent accumulation of this chemically reactive aldehyde occurs during ethanol oxidation, the reaction between acetaldehyde and hepatic macromolecules is a distinct possibility. This reaction would result in a covalent bond, and this process can be studied by accepted radiochemical techniques. Demonstration of covalent binding of acetaldehyde to hepatic proteins will be initially conducted in an in vitro system (liver slices and/or isolated hepatocytes). Once binding of acetaldehyde to hepatic proteins has been established, more detailed investigations will be made into the subcellular locations of this binding. Since it has been shown that mitochondria are structurally and functionally damaged during ethanol metabolism, it would be logical to expect to find acetaldehyde bound to mitochondrial proteins. The nature of the covalent binding and factors controlling it are also important considerations, and finally the confirmation that the observed covalent binding correlates with impaired hepatic structure and/or function is critically important in the verification of our proposed hypothesis. These studies hopefully will give valuable information concerning the basic molecular mechanisms of ethanol-induced hepatotoxicity and suggest a possible rational approach to therapy.